Microvascular injury, without denudation of endothelium is produced with two different techniques (1) a light/dye technique employing a mercury lamp and intravascular sodium fluorescein (2) a He-Ne laser in the presence of Evan's blue. Even without endothelial denudation the endothelial injury produces local platelet aggregation and alters the tone (diameter) of the injured microvessels. The endothelial injury also changes the normal dilating action of acetylcholine to a constriction. The latter suggests interference with an endothelial dependent relaxing factor (EDRF) and demonstrates the probable existence of such a factor in brain microvessels. The effects just described are monitored in pial vessels of mice using intravital microscopy. The mechanisms responsible for the local platelet aggregation and the altered tone will be investigated using pharmacologic probes interacting with the following: cyclooxygenase-prostaglandin-thromboxane pathway, with prostacyclin, with lipoxygenase, with calcium channels. We will attempt to mimic responses to injury by using agents producing radicals or thromboxane. We also will couple the demonstration of endothelium dependent relaxing factor(s) in the microcirculation, with an analysis of the nature of the factor(s) through the use of pharmacologic probes. Finally, in our analysis of factors interferring with platelet aggregation in response to endothelial injury, we will separately study platelet aggregation in vitro, and vessel walls using electromicroscopy. These latter studies will begin to determine whether drugs that affect the response of platelets in vivo, are actually working on the platelet or whether they are, instead or also, altering the amount of injury at the vessel wall. The proposed studies are relevant to important clinical problems such as (1) cerebral infarction due to small vessel obstruction following endothelial damage (2) hypertrophy of arteriolar walls which might be dependent upon release of smooth muscle mitogens during enhanced platelet aggregation (reported by some to occur in hypertension) (3) altered vascular reactivity following endothelial injury in such diverse conditions as ischemia, hypertension, vascular rupture with subarachnoid hemorrhage, trauma.